A 94-year-old former SS sergeant has told a German court he is "ashamed" that he served as a guard in the Nazis' Auschwitz death camp.

Reinhold Hanning apologised to Holocaust survivors looking on in the courtroom for doing nothing to try to stop what was going on, even though he was aware Jews were being gassed and their corpses burned.

He told the Detmold state court he had never spoken about his service in Auschwitz from January 1942 to June 1944, even to his family, but wanted to use his trial as an opportunity to set the record straight.

Sitting in a wheelchair and speaking into a microphone with a weak voice, he said: "I want to say that it disturbs me deeply that I was part of such a criminal organisation.

"I am ashamed that I saw injustice and never did anything about it and I apologise for my actions. I am very, very sorry."

As he spoke, Auschwitz survivor Leon Schwarzbaum watched from around five metres away with a steely face.

He said afterwards he was happy that Hanning apologised but it was not enough.

The 95-year-old said: "I lost 35 family members, how can you apologise for that?

"I am not angry, I don't want him to go to prison but he should say more for the sake of the young generation today because the historical truth is important."

Hanning is charged with 170,000 counts of accessory to murder on allegations that as a guard he helped the death camp function, so can legally be found guilty of accessory to murder.

Prosecutor Andreas Brendel said there was good evidence already that Hanning served in the camp, but that his admission on Friday could help win a conviction.

He said: "Today's statement contributed a little more to establish that he was there, because he admitted that, and more importantly to the fact that he knew about the killings in the main camp - that also is a crucial fact."

Pleas are not entered in the German system and such statements to the court are not uncommon, and frequently help mitigate the length of a sentence.

Hanning faces a possible 15 years in prison if convicted, but at his age it is unlikely he will ever spend time behind bars given the length of the appeals process.

Ahead of the short statement he made himself, Hanning's lawyer Johannes Salmen read a 22-page statement from Hanning detailing how his client had joined the Hitler Youth with his class in 1935 aged 13, then volunteered at 18 for the Waffen SS in 1940 at the urging of his stepmother.

He fought in several battles before being hit by grenade splinters in his head and leg during close combat in Kiev in 1941.

Hanning spoke fondly of his time at the front and said as he was recovering from his wounds he asked to be sent back but his commander decided he was no longer fit for frontline duty, so sent him to Auschwitz.

He said he did not know what Auschwitz was at that time, but quickly found out, though he said his initial responsibility was to register patrols and work details coming and going through the front gate, far away from where the killings were taking place.

"Nobody talked to us about it in the first days there, but if someone, like me, was there for a long time then one learned what was going on," he told the court in the statement, looking down at the table in front of him as it was read aloud.

"People were shot, gassed and burned. I could see how corpses were taken back and forth or moved out. I could smell the burning bodies; I knew corpses were being burned."

He was later assigned to a guard tower and said all guards had orders to shoot prisoners trying to escape.

However, he did not say whether he ever shot anyone himself and did not mention any specific involvement in the killings in Auschwitz, where nearly one million Jews and tens of thousands of others were slaughtered.

"I've tried my whole life to forget about this time," he said. "Auschwitz was a nightmare."